The present invention generally relates to fabrication of semiconductor devices and more particularly to a method of fabricating a semiconductor device including a step of eliminating electric charges from the semiconductor device together with a carrier used for holding the semiconductor device which is under fabrication.
Generally, the process of fabricating a semiconductor device includes steps such as cleaning, storing and transporting semiconductor wafers on which the semiconductor devices are to be formed. During the foregoing steps, the semiconductor wafers are accommodated in a container called a carrier which has an appearance similar to a basket. While various materials such as silica are employed for the carrier, perfluoroalcoxy resin referred to hereinafter as PFA is particularly preferred because of the extremely low content of impurities which may otherwise cause contamination to the wafer, excellent stability in the high temperature environment, easiness of molding, and resistance against forming dusts even when contacted with the semiconductor wafer. On the other hand, the PFA carrier has a problem in that it tends to accumulate electric charges because of its high resistivity. For example, when the PFA carrier is lifted from a deionized water after immersion in the water for cleaning, electric charges of about -10,000 volts is accumulated on the PFA carrier. Once the carrier is charged as such, it takes about one hundred hours or more until the electric charges disappear as long as the discharge is made by a natural discharge process. When the carrier is charged electrically, the dusts in the environment are attracted to the carrier and thus to the semiconductor wafer held thereon. Note that the semiconductor wafers, too, have a thin oxide film on the surface and thus accumulates the electric charges when the PFA carrier is charged as such. This problem of attraction of dusts is particularly serious for the fine dusts having sub-micron particle diameters. In the conventional semiconductor devices designed for relatively low integration density with relatively coarse semiconductor patterns, this problem of the PFA carrier attracting dusts does not cause any critical problems, although such attraction is certainly not desirable. On the other hand, in the case of the semiconductor integrated circuits having a high integration density, the dusts having a particle size as small as about 0.05 .mu.m are not tolerable. This value of particle size corresponds to about one-tenth of the width of the lines formed on the surface of the wafer by patterning. Thus, the problem of charging of the PFA carrier raises a serious problem in the fabrication of the semiconductor devices having extremely fine patterns.
In order to eliminate the foregoing problem, there is a proposal to incorporate carbon particles into the PFA forming the carrier. As a result of incorporation of the carbon particles, the PFA carrier is provided with a finite conductivity and a quick elimination of the electric charges is expected. However, this approach has a problem in that carbon particles introduced in PFA tends to enhance the reaction between PFA and chemical agents such as nitric acid used for cleaning and thereby PFA forming the carrier is deteriorated. Further, such a PFA carrier admixed with carbon particles tends to create dusts of carbon which are detrimental to the operation of the semiconductor device.
Alternatively, there is proposed an approach wherein the electric charges are eliminated by applying a surfactant on the surface of the PFA carrier. However, this approach, utilizing the surfactant which may contain impurities, increases the risk that the semiconductor device is contaminated by the impurities.
Alternatively, there is proposed another approach wherein the electric charges are eliminated from the semiconductor device as well as from the PFA carrier, by placing the semiconductor device and the PFA carrier in a stream of ionized air. This approach, however, requires an extra step wherein a high voltage has to be applied to produce the ionized air, and is therefore not desirable from the view point of safety and easiness of implementation.